Feeding behavior and performance of Neodiprion sertifer larvae reared on Pinus sylvestris needles

The feeding behavior of herbivorous insects is a crucial element of plant-insect interactions. The quality of food often determines the place of feeding and performance of herbivorous insects. The aim of the study was to show how food quality controls feeding behavior of European pine sawfly (Neodiprion sertifer Geoffr.), an important pest of young pine plantations. In spring 2003 we observed an outbreak of N. sertifer on 10-year-old Pinus sylvestris L. trees. We found that the larvae fed mainly on 1-year-old needles belonging to the branches where eggs were laid during the previous year. The older (2-year old) and younger (current-year) needles and the needles from the terminal leader (trunk) were eaten by larvae only after all 1-year-old needles were consumed. We conducted two experiments with larvae of European pine sawfly. In the first experiment (May 2003) larvae were released on the first, second or third whorl of Scots pine trees. After 1 and 2 days we recorded the locations where larvae fed. In the next experiment (in May 2004) we reared the larvae in the laboratory using four needle types: 1-year old from branches, 2-year old from branches, 1-year old from terminal leader and 2-year old from terminal leader. In the first experiment larvae passed by the 2-year-old needles with higher content of phenolic compounds and fed on 1-year-old needles. In most cases the larvae fed on the branches and only incidentally on the terminal leader (χ², P < 0.01). The age and type of needles affected duration of insect development and efficiency of ingested food. The duration of development was shorter, the amount of food eaten smaller and the efficiency of ingested food higher, in the larvae raised on 1-year-old needles from branches. Female individuals were twice as big as males, therefore they had longer duration of development and ate more food. The level of total phenolic compounds and nitrogen were significantly different by age and type of needles. Our results indicated that larvae raised on needles high in nitrogen and low in phenolic compounds were characterized by short duration of development. Therefore, it is likely that the chemical composition (food quality) of Scots pine needles, not accident, is responsible for the choice of feeding place for European pine sawfly larvae.